X-ray reflectometry (XRR) is a well-known technique for measuring the thickness, density and surface quality of thin film layers deposited on a substrate. Conventional X-ray reflectometers are sold by a number of companies, among them Technos (Osaka, Japan), Siemens (Munich, Germany) and Bede Scientific Instrument (Durham, UK). Such reflectometers typically operate by irradiating a sample with a beam of X-rays at grazing incidence, i.e., at a small angle relative to the surface of the sample, near the total external reflection angle of the sample material. Measurement of X-ray intensity reflected from the sample as a function of angle gives a pattern of interference fringes, which is analyzed to determine the properties of the film layers responsible for creating the fringe pattern. The X-ray intensity measurements are commonly made using a position-sensitive detector, such as a proportional counter or an array detector, typically a photodiode array or charge-coupled device (CCD).
A method for analyzing the X-ray data to determine film thickness is described, for example, in U.S. Pat. No. 5,740,226, to Komiya et al., whose disclosure is incorporated herein by reference. After measuring X-ray reflectance as a function of angle, an average reflectance curve is fitted to the fringe spectrum. The average curve is based on a formula that expresses attenuation, background and surface roughness of the film. The fitted average reflectance curve is then used in extracting the oscillatory component of the fringe spectrum. This component is Fourier transformed to find the film thickness.
U.S. Pat. No. 5,619,548, to Koppel, whose disclosure is incorporated herein by reference, describes an X-ray thickness gauge based on reflectometric measurement. A curved, reflective X-ray monochromator is used to focus X-rays onto the surface of a sample. A position-sensitive detector, such as a photodiode detector array, senses the X-rays reflected from the surface and produces an intensity signal as a function of reflection angle. The angle-dependent signal is analyzed to determine properties of the structure of a thin film layer on the sample, including thickness, density and surface roughness.
U.S. Pat. No. 5,923,720, to Barton et al., whose disclosure is incorporated herein by reference, also describes an X-ray spectrometer based on a curved crystal monochromator. The monochromator has the shape of a tapered logarithmic spiral, which is described as achieving a finer focal spot on a sample surface than prior art monochromators. X-rays reflected or diffracted from the sample surface are received by a position-sensitive detector.
Various types of position-sensitive X-ray detectors are known in the art of reflectometry. Solid-state arrays typically comprise multiple detector elements, which are read out by a CCD or other scanning mechanism. Typically, each element accumulates photoelectric charge over a period of time before being read out and therefore cannot resolve the energy or number of incident X-ray photons. XRR systems known in the art that are based on such arrays simply record the total integrated radiation flux that is incident on each element. The signals at low angles, below the total external reflection angle, are usually much stronger than the signals above this angle. A ratio of 105 to 107 in photon flux between 0° and 3° reflections is typical. The dynamic range of array detection systems known in the art is substantially smaller than this ratio. Consequently, high-order fringes at higher incidence angles cannot generally be detected. Photon counting sensitivity is needed in order to measure the weak signals at these angles.
A further drawback of X-ray thin film measurement systems known in the art is their lack of spatial resolution. X-ray optics, such as the above-mentioned curved monochromators, are capable of focusing an X-ray beam to a spot diameter below 100 μm. When the beam is incident on a surface at a low angle, below 1°, for example, the spot on the surface is elongated by more than 50 times this diameter. A measurement that is made under these circumstances provides only an average of surface properties over the entire elongated area. For many applications, such as evaluating thin film microstructures on integrated circuit wafers, better spatial resolution is required.
Although the present patent application is concerned mainly with systems in which a sample is irradiated by a monochromatic beam, other methods for X-ray reflectometry are also known in the art. One such method is described, for example, in an article by Chihab et al., entitled “New Apparatus for Grazing X-ray Reflectometry in the Angle-Resolved Dispersive Mode,” in Journal of Applied Crystallography 22 (1989), p. 460, which is incorporated herein by reference. A narrow beam of X-rays is directed toward the surface of a sample at grazing incidence, and a detector opposite the X-ray beam source collects reflected X-rays. A knife edge is placed close to the sample surface in order to cut off the primary X-ray beam, so that only reflected X-rays reach the detector. A monochromator between the sample and the detector (rather than between the source and sample, as in U.S. Pat. No. 5,619,548) selects the wavelength of the reflected X-ray beam that is to reach the detector.